1. Field of the Invention
The present invention relates to a balance with a weighing cell which is surrounded by an elastic tubular bellows and is protected against the penetration of dust and water, wherein one end of the bellows is attached to the stationary end of the weighing cell and the other end of the bellows is attached to the load-receiving deflectable end of the weighing cell.
2. Description of the Related Art
Balances are not only used in dry rooms which contain a small amount of dust, but also in industrial plants and manufacturing plants, such as butcher stores, slaughter houses, chemical plants, etc. Particularly in plants of the food industry, regulations require frequent cleaning of all machines and devices and, consequently, also of the balances, with a certain liquid, such as water. This cleaning takes place in many cases with a high pressure cleaning apparatus at whose nozzle tip pressures of up to 150 bars may occur.
Accordingly, sealing units for balances have already been proposed which have the purpose of preventing the penetration of spray water into the measuring cell. In a balance disclosed in U.S. Pat. No. 4,957,177, the weighing or measuring cell is placed in a bellow pipe of metal. The ends of the bellow pipe are attached to the stationary part of the weighing cell, on the one hand, and to the load-receiving, deflectable part, on the other hand. This known bellows protects the weighing cell without problems against the penetration of dust and liquids. However, the configuration of this known bellows requires that the wall thickness of the bellows is very thin, so that the bellows has a soft-elastic behavior and, thus, the influence on the weighing result, for example, due to hysteresis, is kept as low as possible.
Bellow pipes of steel having very thin walls can be manufactured economically only with small diameters. However, these types of bellows have additional disadvantages. Thus, further processing of the guide members is required after the assembly in order to avoid errors due to eccentric loads. This means that the weighing cell becomes more expensive or can only be used in balance constructions in which the eccentric load is compensated in another manner. Moreover, the manufacturing costs for a bellows of metal are very high, particularly when it is of a stainless material.
It is also known in the art to use a rubber bellows instead of a bellows of metal. Such a rubber bellows has the necessary soft elasticity and, in addition, can be manufactured inexpensively. However, the rubber bellows has the disadvantage that its resistance against the hard jet of a high pressure cleaning apparatus is very low and, therefore, the service life of the rubber bellows is short. Moreover, a bellows made of rubber very quickly buckles inwardly under the influence of the water jet and may be damaged by the edges of the measuring cell. In addition, the bellows may not return into its initial position by itself. The buckling of the bellows into bistable positions additionally changes the bypass force emanating from the bellows and influencing the weighing result.
DE-C2-42 20 810 discloses a load receiving means with a beam-shaped weighing cell which is provided with a wire strain gauge arrangement. For damping the vibrations of the balance, the stationary part and the deflectable part of the measuring cell are connected to a tubular metal bellows which surrounds a rod projecting from one end of the weighing cell and a tube which projects from other end of the measuring cell and concentrically surrounds the rod. This bellows is filled with oil as a damping medium. Together with the parts which protrude into each other, i.e., the rod and the tube, the desired damping is achieved by the volume-constant configuration of the metal bellows. The bellows does not surround the measuring cell, but is located within or between the two guide members which extend parallel to each other and connect the two ends of the weighing cell. Consequently, the weighing cell itself and the wire strain gauges mounted thereon are not protected against external influences.
DE-C3-17 74 653 discloses a support means for a bridge of an electromechanical weigh bridge in which the measuring cell is protected against the penetration of moisture and dirt by means of two elastic bellows and a metal pipe piece arranged between the two bellows. There are no measures taken against buckling of the bellows which are not identified in detail. Cleaning of the measuring cell by means of a high pressure cleaning apparatus is rarely carried out in a weighbridge.